Various embodiments of an attenuator are described herein. In particular, the embodiments described herein relate to an improved attenuator for use in a vehicle braking system and a vehicle braking system equipped with the attenuator.
Devices for autonomously generating brake pressure have been a part of the prior art since the introduction of driver assistance functions, such as, for example, a vehicle stability control (VSC), and are being built into vehicles during series production. Autonomously generating brake pressure makes it possible to brake individual wheels or all wheels of the vehicle independent of the driver actuating the brake. In the meantime, additional driver assistance functions beyond the safety-related VSC have been developed to the point of readiness for series production which assume safety functions as well as comfort functions. Adaptive cruise control (ACC) is a part of this for example.
When the ACC function is activated, the distance and relative speed of a vehicle traveling up ahead is recorded by laser distance sensors or preferably radar distance sensors. Like conventional cruise control, the ACC function maintains a speed selected by the driver until a slower vehicle traveling up ahead is registered and a safe distance from it is no longer being maintained. In this case, the ACC function engages by braking to a limited extent and if needed by subsequent acceleration in order to maintain a defined spatial or temporal distance from the vehicle traveling up ahead. Additional ACC functions are expanded to the extent of also braking the vehicle to a stop. This is used for example in the case of a so-called follow-to-stop function or a function to minimize a collision.
Further developments also permit a so-called stop-and-go function, wherein the vehicle also starts automatically if the vehicle up ahead is set in motion again. To do so, the stop-and-go function must be able to execute a frequently changing autonomous pressure build-up to approx. 30 to 40 bar in the vehicle braking system independent of the generation of brake pressure originating from the driver. In the case of typical speeds on freeways, an autonomous deceleration is often restricted to approx. 0.2 g, at lower speeds, on the other hand, the system can generate an autonomous deceleration of 0.6 g for example. A further development also includes an automatic emergency brake (AEB), whereby the ACC function detects potential accident situations in due time, warns the driver and simultaneously initiates measures to autonomously brake the vehicle with full force. In this case, rapid pressure build-up rates to brake pressures of approximately 100 bar and greater are required.
Correspondingly powerful devices for autonomously generating brake pressure include pumps, such as piston pumps, which can be annoying sources of noise. In particular the conveyance of brake fluid through piston pumps generates pulsations, which can spread audibly via brake circuits and also affect the noise level in the vehicle's interior.
To dampen noises or pulsations, devices for autonomously generating brake pressure are known that feature a throttle on the outlet side of the pump. U.S. Pat. No. 5,540,486 shows, in FIG. 1 for example, a pump 24 with an attenuator 26 arranged downstream from same and an orifice 28. Printed document WO 02/14130 A1 shows a vehicle braking system, which comprises a device for autonomously generating brake pressure with a pump 8, a compensating tank 48 arranged downstream from the pump and a throttle 49. Namely by using a throttle, the pump noises can be damped and an improvement in comfort is achieved, however, the throttle has a limiting effect on the pressure build-up rates.
The use of attenuators which reduce amplitude of pressure fluctuations in hydraulic fluid lines of vehicular braking systems is well known. In particular, attenuators are common in vehicular anti-lock braking systems (ABS) at the outlet end of an ABS hydraulic pump used to evacuate the low pressure accumulator. A hydraulic control unit (HCU) includes a housing having bores for mounting valves and the like and channels for directing fluid. An attenuator is mounted in a bore in the HCU to significantly reduce the amplitude of high energy pressure pulses in the brake fluid at the outlet of the pump. Such pressure pulses can create noise which is transmitted to the master cylinder or its connection to the vehicle.
One known attenuator includes a closed chamber filled with brake fluid. An inlet passage delivers fluid from the outlet end of the pump. An orifice of substantially reduced diameter directs fluid from the chamber to an outlet passage. The restriction of fluid flow through the orifice attenuates pressure fluctuations as a result of the compressibility of the brake fluid. Thus, brake fluid in the chamber absorbs high energy fluid pulses and slowly releases the fluid through the orifice.
Another known attenuator for use in an ABS system is disclosed in U.S. Pat. No. 5,540,4306 to Linkner. The attenuator 26 includes an elastomer core piece 410′. The core piece 410′ includes an annular seal 66′ at the head end 412′ of the attenuator and an axially extending compression rib 52′.
Another known attenuator for use in an ABS system is disclosed in U.S. Pat. No. 5,921,6404 to Roberts. The attenuator 70 includes a cylinder 72 slidably received in a bore 73 of the housing 400. A cap 74 is integrally formed with the cylinder 72. An elastomeric plug 80 is received in the cylinder 72. The plug 80 has a shape complementary to the tapered interior surface of the cylinder 72. An annular groove 86 is formed in an outer surface of the plug 80. The inner end of the plug 80 includes an inwardly projecting stem 88 which engages a bottom wall of the bore 73.
To achieve the pressure build-up rates required for driver assistance functions, a more powerful pump can be connected upstream from the throttle. However, the manufacturing costs of the vehicle braking system increase with the higher pumping capacity, which stands in the way of using the driver assistance functions in more economically priced vehicles. In addition, a throttle can significantly reduce the service life of the pump or disproportionately increase the load on the vehicle's electrical system through higher motor currents.